This invention relates to electronic drive circuits for solenoids, and more particularly to such an electronic drive circuit that utilizes a pulse width modulated technique.
A solenoid is an electromechanical device that transduces or changes an electrical signal, which is input to the wire coil of the solenoid, into a corresponding mechanical movement of a metallic device, such as a rod, disposed within the coil. The electrical current flowing through the wire coil creates a magnetic field that either attracts or repels the metallic device. The metallic device is typically connected to a mechanical device, such as an actuator, which is physically moved along with the metallic device of the solenoid by the magnetic field.
Solenoids are commonly used in a wide range of both commercial and military devices. For example, solenoids are used on aircraft to control various mechanical devices and variables.
Various electronic circuits are used for driving or controlling the solenoid coil. See, for example, U.S. Pat. Nos. 4,381,532, 4,546,403, 4,556,926, 4,764,840, 4,949,215 and 5,345,181. A typical circuit comprises a solenoid being connected in series with a supply voltage, a transistor and a sense resistor. The voltage across the sense resistor is indicative of the current flowing through the solenoid coil. It is usually required to control the current through the solenoid such that it does not exceed a certain value, else the solenoid would fail.
The voltage across the sense resistor is typically fed to one input of a comparator, the other input of which is fed a reference voltage. If the sense resistor voltage exceeds the reference voltage, the comparator output toggles or switches state. Subsequent signal processing circuitry downstream from the comparator controls the switching of the transistor in series with the solenoid to its off state. This prevents an over-current condition in the solenoid coil.
Accordingly, it is a primary object of the present invention to provide an electronic drive or control circuit for a solenoid, the circuit utilizing a pulse width modulated scheme.
It is a general object of the present invention to limit the electrical current flowing in a solenoid coil to a predetermined maximum value that allows for proper solenoid operation.
It is another object of the present invention to sense an open coil condition of the solenoid.
It is yet another object of the present invention to utilize a pulse width modulated scheme for driving a solenoid such that the scheme operates on repetitive time periods or windows, with the solenoid coil having a current applied thereto for a portion of each window, regardless of whether or not an over-current condition exists in the solenoid.
The above and other objects and advantages of this invention will become more readily apparent when the following description is read in conjunction with the accompanying drawings.